In the propagation of microwave signals, it is generally desired to confine the signals to one propagation mode in order to avoid the distortions that are inherent in multimode propagation. The desired propagation mode is usually the dominant mode, such as the TE.sub.11 mode in circular waveguide. The higher order modes can be suppressed by careful dimensioning of the waveguide such that the higher order modes are below cutoff. In certain instances, however, it is necessary for portions of the waveguide to be large enough to support more than one mode, and a discontinuity in such a waveguide can give rise to undesired higher order modes. For this reason, such waveguide sections are often referred to as "multi-mode" or "overmoded" waveguide.
One example of a waveguide system that requires an overmoded waveguide section is a system that includes a multi-port, multi-frequency combiner. For example, four-port combiners are typically used to permit a single antenna to launch and/or receive microwave signals in two different frequency bands in each of two orthogonal polarizations. Each of these frequency bands is usually at least 500 MHz wide. For instance, present telecommunication microwave systems generally transmit signals in frequency bands which are referred to as the "4 GHz", "6 GHz" and "11 GHz" bands, but the actual frequency bands are 3.7 to 4.2 GHz, 5.925 to 6.425 GHz, and 10.7 to 11.7 GHz, respectively. Signals of a given polarization in any of these bands must be propagated through the combiner without perturbing signals in any other band, without perturbing orthogonally polarized signals in the same band, and without generating unacceptable levels of unwanted higher order modes of any of the signals.
Elaborate and/or costly precautions have previously been taken to avoid the discontinuities that could give rise to undesired higher order modes in multi-frequency combiners of the type described above. For example, U.S. Pat. No. 4,077,039 discloses such a combiner that uses a pseudo-balanced feed in the tapered portion of a flared horn, in combination with evanescent mode waveguide filters in the side arms of the high frequency port of the combiner. The basic dilemma posed by the multi-port, multi-frequency combiners is the undersired mode-generating discontinuities must be avoided in the overmoded waveguide sections, and yet some means must be provided for coupling selected signals with one or more ports located in the overmoded section of waveguide. Previous solutions of this dilemma have involved various complex, costly and/or physically cumbersome designs.